One of the problems accompanying the explosive growth in the use of micro-chips, circuit boards, circuit cards, multi-chip modules, power supplies/converters and power amplifiers and other heat generating parts (hereinafter referred to as "components") is the necessity for dissipating heat generated by these electronic components. In order for the components to function most efficiently and to reduce the risk of malfunction, the electronic components must be maintained in certain desired temperature ranges.
A number of methods for cooling electronic components are currently in use. Placing fans in the housing containing the electronic components and forcing air across the components is one of the most common cooling solutions. However, different electronic components produce different quantities of heat during operation, leading to hot spots. Heat sinks have been used to combat the hot spot problem. However, as the wattage output of electronic components has continued to increase, in order to achieve the desired cooling the heat sinks have become excessively large. External fins have been incorporated on the housing containing the electronic components in an attempt to dissipate the heat build up inside. In sophisticated systems, cooling tubes containing a cooling fluid have been routed around the electronic components in order to conduct the heat to an external heat exchanger. However, cooling tubes and heat exchangers are space intensive and expensive in relation to the present invention.
Electronic hardware manufacturers have introduced sophisticated systems for circulating liquid coolant in the component housing directly around and in contact with the electronic components and then re-circulated back into the housing. The liquid coolant is collected and conducted to a heat exchanger/condenser, usually outside the housing of the electronic components. Such systems are relatively expensive and space intensive as well. Additionally, such systems are inefficient in that they involve single phase cooling.
Much commercial research and development of liquid cooling systems for electronic components and other heat generating parts has been concentrated within the related field of jet impingement of liquid coolants, consisting of a high velocity, narrow jet or jets of liquid directed upon the surface to be cooled. Jet impingement may be confused with spray cooling, particularly in that a liquid coolant is discharged from an orifice and directed at the cooled surface in both cooling methods; however, there are many fundamental and significant differences in the fluid dynamics and heat transfer mechanisms between the impingement of a fluid jet and the impingement of a well-dispersed and atomized spray of liquid droplets over a much larger area. Jet impingement cooling has limitations rendering it inferior to spray cooling. It does not provide uniformity of cooling over the surface, requires higher flow rates for an equivalent average heat flux, and burns out (transition to vapor film boiling with consequent increase in surface temperature) at lower critical heat fluxes (CHF) than spray cooling. When cooled by a jet, the outer region of an area transitions to film boiling (hence, where CHF occurs) at a relatively low heat flux, due to the lower heat transfer coefficients. This both reduces the heat removal in these areas, and increases the local surface temperature. This instantly places an incrementally larger heat removal burden on the inner areas which cannot be accommodated because there is no corresponding incremental increase in heat transfer capability. Thus the film boiling phenomena quickly travels radially inward.
In order to achieve better cooling with jet impingement, attempts have been made to increase the number of jets and thereby decrease the area cooled by each jet. However, the benefits are quickly offset by the geometrically increasing difficulty of delivering and removing large quantities of fluid consequently jet impingement becomes impractical for misting large areas. As will be made clear in the following description, the present invention does not suffer from these limitations. Within the field of single phase jet impingement cooling a number of patents have been issued including U.S. Pat. Nos. 4,108,242; 4,912,600; 4,838,041 and 3,844,343.
To achieve more efficient cooling there have been numerous investigations and attempts into the capabilities of spray cooling. Spray cooling includes the additional advantage of an evaporative phase change. A number of patents including U.S. Pat. Nos. 4,643,250; 4,790,370; 4,643,250; 4,352,392 and 4,967,829 provide insight into the prior art of evaporative cooling. Generally, the prior art has not been successful because either conventional atomizers are unsuitable for viable spray cooling or large spraying distances and system volumes lead to inefficiencies.
U.S. Pat. No. 5,220,804 discloses an atomized liquid sprayed across a wide distribution that impinges upon the surface of the electronic components to be cooled. The heat is transferred to the coolant in an evaporative phase change process. The vapor and liquid is collected and removed to an external condenser. While such a system incorporates the advantage of evaporative phase change cooling, the process of evaporation and condensation is not self contained with in the heat generating components' housing, the liquid coolant and vapor being collected and recirculated by an external pump and condenser. In many applications, including telecommunications, avionics and military, it is very desirable to have self contained units without any external systems. Such self contained systems are advantageous in space constrained systems and avoid potential EMI problems. Additionally, while the atomizers of U.S. Pat. No. 5,220,804 are superior in many respects to previous prior art atomizers in performance, the atomizers of U.S. Pat. No. 5,220,804 are not adaptable to all applications.